*Earth’s nearest neighbor in the universe is the moon!  After today’s lecture, you should be able to:

• Explain various hypotheses about how the moon formed.
• Describe features and properties of the moon.

Astronomy: the study of the universe

• The 1^st probes landed on the moon in 1959.
• The 1^st people landed on the moon in 1969, a mere 10 years later!
• As a result of these missions, lunar rock has been brought to Earth, which has enabled us to learn more about the history of the moon and the Earth!

There are 4 main theories about the moon’s origin:

1. The Earth and moon formed at the same time, w/the moon in orbit around Earth.
2. Early Earth was spinning so fast, that a chunk of it spun off into orbit.
3. The moon formed elsewhere in the solar system and was later captured by Earth’s gravitational pull.
4. The moon formed about 4.5 billion years ago as a result of a collision between Earth and a planet-sized object.

• This is the most widely accepted theory! (“The Impact Theory”)
• After getting hit, debris from the impact settled into orbit around Earth, and its gravity started to pull it together to form the moon.

Properties and Features of the Moon

*The moon turns on its axis in the same amount of time it takes to complete 1 orbit around the Earth, thus, the same side of the moon always faces Earth!

• 1/4 the Earth’s diameter
• 1/8 the Earth’s mass, thus causing the moon to have less gravity! (1/6 the strength of Earth’s gravity!)
• Lower density than the Earth
• For the last 3 billion years, the interior of the moon has been relatively quiet. 
  • Iron core is very small and not hot enough for large convection cells to carry out plate tectonics!

• Because according to the Impact Theory, the moon formed from the less-dense materials that made-up the Earth’s outer layers!

*When viewed from Earth, the moon appears as a pattern of light and dark areas.

• Dark areas are known as maria, meaning seas, because early observers thought they were looking at water.
• Rille: a trenchlike valley running through maria bedrock formed by old rivers of molten lava
• Lighter areas of the moon are known as lunar highlands, and appear brighter b/c rocks are lighter-colored and thus reflect more sunlight.
• no atmosphere to slow down flying particles
• weak gravity pulling flying particles downward
• Lunar highlands contain the oldest lunar rock, ranging in age from 4.0-4.5 billion years old.
• These lunar rocks help support the theory that the solar system (and thus the Earth and moon) formed about 4.6 billion years ago as a result of a swirling cloud of dust and debris.
  • Craters: depressions in the moon’s surface formed by the impact of meteoroids
  • Rays: bright streaks of shattered rock and dust that radiate out from a crater
  • The moon today mainly gets hit by micrometeoroids– tiny objects no larger than sand grains
  • Why doesn’t the Earth get hit by these?
    • Earth’s atmosphere burns them up!
      • Lunar soil is not really soil, but broken up rock material and dust called regolith.
      • Contains no water, no organic material, and is formed by the smashing impact of meteoroids of all sizes.

• Maria are flat, level basins and plains formed when the moon was still hot and molten and lava spewed up to the surface and cooled.
  • Maria contain the youngest lunar rocks, ranging in age from 3.1-3.8 billion years old.
    • Remember, there is no ‘rock cycle’ on the moon, so rocks can not be recycled and new rocks never form!

• Most lunar highlands lie at the edge of maria.
  • Formed by the impact of meteoroids (rocky or icy fragments that travel through space), which thrust them up!
  • How could large masses of rock be thrust up high enough to form mtns?